The present application relates to printed conductive circuits formed by additive methods and, more particularly, to a novel additive printed circuit process yielding conductors having increased adhesion and resolution capability.
Many types of electronic apparatus are known in which the various electrical components are interconnected by conductors. The interconnecting conductors are fabricated by a wide variety of processes such as, for example, thick-film fired conductor systems, polymer conductors and printed circuit boards.
In thick-film fired conductors, a mixture of a conducting metal powder, a ceramic or glass binder and an appropriate vehicle is screen printed on a substrate. The conductor pattern on the substrate is then fired at a relatively high temperature, typically between 650.degree. and 900.degree. C. As the temperature rises to the firing temperature, the vehicle is volatilized, leaving the metal and binder behind. At the firing temperature, sintering of the metal takes place to a greater or lesser extent with the binder providing adhesion between the metal film formed and the substrate.
Thick-film fired conductors have classically employed precious metals such as gold, silver, platinum and palladium. Recently these noble metals have soared in cost, and new conductor systems using copper, nickel and aluminum are being made commercially available. The cost of the precious metal systems is prohibitive where a low cost conductor system is desired. The newer metal systems are not significantly cheaper because of the special chemistry which is required to prevent oxidation of the metal during the firing process. Moreover, these systems are very difficult to solder using the conventional tin/lead solder and the high firing temperatures required during fabrication preclude the use of low cost substrate materials. Some of the nickel systems can be fired on a soda-lime glass at temperatures just below the melting point of the glass but the resulting conductivity of the conductor is relatively low.
The term "polymer conductor" is actually a misnomer since the polymer is not actually a conductor. Instead, the polymer is heavily loaded with a conducting metal and screened onto a substrate. The advantage of this system is that the polymer can be cured either catalytically or thermally at temperatures which range from room temperature to about 125.degree. C. As a result of this so-called "cold precessing", it is possible to use very inexpensive substrates such as films of Mylar.RTM. (polyethylene terephthalate). The mechanism by which conductivity is achieved is supplied entirely by contact between individual metallic particles. It has been found that the only metals which can be loaded into the polymer and give acceptable conductivity are the precious metals such as gold and silver. All of the other standard conducting metals oxidize over a period of time, reducing the conductivity between the particles. Silver has been the predominant choice in polymer conductor systems but the silver systems are generally not solderable because the silver is leached by the lead-tin solder. When the price of silver is about $10-11 per ounce, these conductor systems are competitive with other systems if used on very low-cost substrates such as thin Mylar.RTM. films. However, when the price of silver is higher, the systems are not competitive with printed circuit boards.
The techniques used to prepare printed circuit boards can be divided into additive, semi-additive and subtractive technlogies. In the semi-additive and subtractive techniques, the starting point is a substrate, which can vary widely from phenolics to glass-filled epoxies, on which a copper foil is bonded. In the traditional additive preparatory system, the copper foil is very thin, usually on the order of about 200 microinches. A resist is patterned such that the copper is exposed only where the conductors are desired and the board is then electroplated to form copper conductors of about 1 mil in thickness. The plating resist is stripped and the copper is etched. In areas where the conductor is not desired, the copper is only about 200 microinches thick so that etching quickly removes this copper while leaving a 1 mil thick conductor. In the subtractive process, the starting thickness of the copper foil is usually between 1 and 2 mils. An etch resist is deposited wherever the conductors are desired, the board is etched and the resist is then removed. The resist prevents etching where the conductors are desired leaving conductor runs.
Both the semi-additive and subtractive printed circuit board procedures require the application of a copper foil over the entire substrate, deposition and removal of a resist, etching of the printed circuit board, drilling holes for component insertion, and in one case, the additional step of electroplating.
In the traditional additive technique, conductors are formed by printing a sensitizer pattern in which the sensitizer contains palladium or a metal which is subsequently replaced by palladium after a sensitizing dip. The substrate is then placed in a catalytic-type plating bath to form an electroless copper or nickel layer in the same area as the sensitizer pattern. This reaction can be allowed to continue until a sufficient thickness of condutor is established. However, this reaction is generally so slow that only a thin layer of conductor is built up in this fashion and the conductor must be subsequently electroplated. The very thin layer of electroless copper initially formed by the catalytic reaction is not capable of sustaining normal plating currents until a substantial additional layer of copper has been built up. If too high a current is applied, a condition generally known as burning results.
The most significant drawback of the printed circuit board technology is that a substantial number of processing steps are necessary and this requires a large amount of associated equipment. In addition, the choice of substrate materials is limited to one of those available for circuit board materials. The number of processing steps and equipment results in relatively high processing costs and the limitation of the substrate material eliminates the opportunity to use a decorative or structural member which may be required in the apparatus as a substrate.
In U.S. patent application Ser. No. 220,342, filed Dec. 29, 1980, now U.S. Pat. No. 4,404,237, issued Sept. 13, 1983 and assigned to the assignee of the present invention, which is entirely incorporated herein by reference, the formation of an electrical conductor by an augmentative replacement reaction technique is described. The desired conductive design is applied to the substrate with an ink composition which contains a finely-divided metal powder, a curable polymer and a solvent. The curable polymer is at least partially cured and then the resulting ink composition pattern is contacted with a metal salt solution in which the metal cation is more noble than the metal of the finely-divided powder and the anion forms a salt with the metal of the salt and the powder which is soluble in the solution. This system is simple to effect; each processing step is relatively fast and the waste materials generated are generally environmentally safe and do not require special disposal processing. The system can be applied to a multiplicity of lowcost substrate materials such as soda-lime glass, plastic and even paper. The augmentative replacement process conductors can be electroplated, as disclosed and claimed in U.S. patent application Ser. No. 336,807, filed Jan. 4, 1982 now abandoned, and assigned to the assignee of the present invention, which application is entirely incorporated herein by reference. The electrical conductors are therein prepared by applying a mixture of metallic powder and a polymer on a substrate, with subsequent curing of the polymer before effecting an augmentative replacment reaction to replace some of the metallic powder with a more noble metal to form a contiguous layer of conducting metal on the substrate and thereafter electroplating an additional metal layer on the contiguous layer.
While the foregoing provides a very low cost process for providing a conductor pattern upon a substrate surface, certain uses require relatively high resolution of the printed conductor lines and the spaces therebetween, typically to 10 milli-inch line widths and spacings. Increased adhesion between the plated conductor layer and the underlying ink, and between the ink and the substrate, while retaining the good thermal and humidity characteristics of the basic ink composition, are also desired. Accordingly, an additive printed circuit process having all of the features set forth in the above-enumerated pair of applications, plus the additional resolution and adhesion properties mentioned, is highly desirable.